The need for providing mounting and interconnection arrangements for the various components and equipment of large scale communication and electronic systems is well known. Components such as circuit packs, printed wiring boards, and the like are physically mounted and electrically interconnected in a manner to facilitate their initial installation and subsequent access for testing, maintenance, and replacement when necessary, for example. One well-known arrangement provides a backplane having large fields of terminal pins extending therethrough to which electrical interconnections between components may be made on either side. On one side, an array of circuit boards, for example, may be mounted in suitable racks and frames and connected to particular arrays of the pins with the other ends of the pins being connected to cabling for electrical interconnections between components. The backplane itself normally also presents printed wiring for making particular electrical connections between terminal pins. Whatever the form of the component mounting arrangements, it will be appreciated that some provision must be made to ensure that the right component is mated with the correct array of backplane pins and, further, that the component is properly polarized with respect to its corresponding pin array. To these ends, a number of coding or keying arrangements have in the past been proposed. One such arrangement is disclosed, for example, in the patent of D. R. Zell, U.S. Pat. No. 3,634,816, issued Jan. 11, 1972, which comprises a pedestal or protrusion extending from a circuit board edge which protrusion is adapted for insertion in a receptacle of a connector. A plurality of stamped inserts are provided which may be selectively fitted about the protrusion in various combinations to fit only in corresponding contours of the connector receptacle. Only one insert combination and thus only a particular circuit board can be mated with a connector. In this arrangement, polarizing, that is, ensuring that the circuit board is not reversed with respect to its connector, may be accomplished by tabs at the ends of the board and connector which mate only with corresponding grooves in the two elements. Although this and other known keying arrangements may be practicable, a simpler and more readily assembled keying arrangement would not only enhance reliability but could achieve significant cost reduction. It is the achievement of these and other objectives to which the keying apparatus of the present invention is chiefly directed.